Many experts believe that a healthful, low-fat diet can help prevent cancer. Observational studies indicate that people who live in cultures that consume a lower-fat diet have a reduced incidence of breast and colorectal cancer . But other research examining this topic has had mixed results.

In two new studies in the February 8, 2006 issue of the Journal of the American Medical Association , researchers report that a low-fat diet did not significantly affect breast or colorectal cancer risk in postmenopausal women. There was some evidence, however, that the diet may reduce the risk of certain types of breast cancer.

About the Study

These studies were part of the Women’s Health Initiative Dietary Modification Trial, a randomized controlled trial that included 48,835 postmenopausal women ages 50-79 years with no history of cancer. The women were randomly assigned to a low-fat diet intervention group or a usual-diet comparison group. The women in the intervention group attended regular group sessions with a nutritionist designed to help them achieve the following goals:

  • Reduce fat intake to 20% of total calories (as opposed to about 35% on the usual-diet)
  • Consume at least five servings of fruits and vegetables per day
  • Consume at least six servings of grains per day

The women in the breast cancer study had mammograms every two years. Those in the colorectal cancer study underwent colorectal screening and diagnosis procedures as recommended by their personal physicians. The researchers followed the women for an average of eight years, tracking whether they had developed breast or colorectal cancer.

Overall, 1,727 women developed breast cancer and 201 developed colorectal cancer. There was no significant difference in the risk of developing either cancer between the low-fat and regular diet groups. However, the low-fat diet was associated with a 24% reduction in risk of developing breast cancers classified as “progesterone receptor negative.” In addition, women who consumed a greater percentage of fat when the study began were up to 22% less likely to develop breast cancer after adopting the low-fat diet.

These studies are limited because women in the intervention group only achieved 70% of the desired reduction in fat. In addition, 10% of the participants did not have colorectal cancer screening during the follow-up period, since this was not a requirement of the study. Finally, all fats are not created equal, and the researchers did not isolate the effects of eating unhealthy fats (e.g., saturated and trans ) versus healthy ones (e.g., polyunsaturated) in these particular studies.

How Does This Affect You?

These findings suggest that a low-fat diet may not prevent breast or colorectal cancer in postmenopausal women. However, the trends seen in the breast cancer study suggest that a low-fat diet may help prevent certain types of cancer and may benefit women who currently consume a higher percentage of fat.

Cancer takes many years to develop, so the eight-year follow-up in these studies may not have been long enough to show a benefit. If you are looking for ways to reduce your risk of developing cancer, the American Cancer Society recommends eating a healthful diet rich in fruits, vegetables, and whole grains. Future research will help determine the optimum diet for cancer prevention, including hopefully the influence of fat quality (e.g., saturated vs. unsaturated) rather than just quantity.